A mobile network, or a radio communication network, such as e.g. a Wideband Code Division Multiple Access (WCDMA) network, a Long Term Evolution (LTE) network, or any other type of mobile network capable of providing wireless network access commonly rely on a large number of geographically distributed base stations or access nodes which are capable of providing network access to communication devices or User Equipments (UE). In order to accomplish this it is a very fundamental requirement of the mobile network that as a communication device move between different cells of the network, it must be capable to hand over an ongoing radio connection, i.e. a voice call or a data session from one base station to another with none or minor disruptions.
A HO can occur between cells arranged within one and the same Radio Access Technology (RAT), e.g. as a HO from one LTE cell to another LTE cell, or as an inter-RAT HO, also referred to as IRAT HO, such as e.g. a HO from an LTE cell to a WCDMA cell, or from an LTE cell to a Universal Mobile Telecommunication System (UMTS), or vice versa. If adapted therefore, a HO may also be executed between a 3rd Generation Partnership Project (3GPP) access technology network, such as e.g. any of the ones mentioned above, and a non-3GPP access network technology, such as e.g. a Wireless Local Area Network (WLAN).
A HO between LTE cells may be referred to as an Intra-LTE HO, which assures that a communication device is being served by the most suitable cell at all times, thereby avoiding the occurrence of call drops, or interruptions, as a communication device is moving out of coverage of one LTE cell into the coverage of another cell. A HO performed in an LTE enabled mobile network is network controlled based upon measurement reports, which may also be referred to as UE measurement reports, provided from a serving cell as well as from neighboring cells.
An intra-LTE HO process is managed by a radio base station, referred to as eNodeB or eNB in an LTE network. FIG. 1 is illustrating a procedure for preparing for and executing an intra-LTE HO, according to a prior art solution, as described in 3GPP TS 36.300. The procedure as described in FIG. 1 can be divided into an initial measurement phase, followed by a preparation phase and a final execution phase.
The initial measurement phase handles measurements as illustrated with steps 1:1-1:10. As indicated in step 1:1 necessary signalling is exchanged between a serving Gateway (GW) 100, A Mobility Management Entity (MME) 110, a source eNB 120, i.e. the eNB presently serving a UE 140, and a target eNB 130, i.e. the eNB to which the radio connection will eventually be handed over. In a next step 1:2 a request for measurements (measurement control) is sent from the source eNB 120 to the UE 140 which need to perform a HO. The measurement control sets different parameters determining certain conditions for the preparation of a HO, to the UE 140, one of which is referred to as a hysteresis, or HO hysteresis, while another is referred to as Time-To-Trigger (TTT). In the meantime packet data is distributed over the network, as indicated with steps 1:3a and 1:3b. The UE 140 responds by making periodic measurements, as indicated with step 1:5, after having performed UL allocation, as indicated with step 1:4. The periodic measurements are indicating the quality of the radio signal, received from the serving cell and adjacent cells, for maintaining the ongoing radio connection and the radio bearers providing the radio connection. Such measurements are commonly referred to as a Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). In case the applied HO algorithm is based on RSRP measurements, an RSRP value from an adjacent cell exceeding the corresponding value from the serving cell by a number of dBs equal to the HO hysteresis will trigger the starting of a timer and the UE will enter a phase which is referred to as Event3A.
Once the timer has started, the process can continue according to one of two scenarios. According to one scenario, the RSRP value of the serving cell can improve during the TTT, so that this value becomes better than the best adjacent cell by a number of dBs equal to the HO hysteresis. In this case the UE 140 will stop the timer with a “Leave Event3A” event, leading to that no HO will be executed. UE 140 will instead be served by the serving cell and nothing more will be reported to the network. According to another scenario the TTT will time out without the serving cell becomes sufficiently better than the best adjacent cell by a number of dBs equal to the HO hysteresis. The UE 140 will then send a measurement report, as indicated with step 1:5 to the source eNB 120, which will evaluate the content of the report and make a HO decision based on the evaluation, as indicated with a subsequent step 1:6. TTT and HO hysteresis are configurable parameters which can be optimized by the network. The preparation phase is now started by the source eNB 120, transmitting a HO request, following the HO decision, to the target eNB 130, in another step 1:7, after which the target eNB 130 responds by executing an admission control 1:8 and acknowledging the request, as indicated with another step 1:9, and making a Downlink (DL) allocation for UE 140, after which the HO execution phase is initiated, as indicated in step 1:11, and the HO is completed in a subsequent step 1:12. The execution phase occurs mainly between the UE 140 and the target eNB 130, involving e.g. synchronization and random access. During this phase there will be an outage time where neither the source nor the target cell can reach the UE 140. Packets arriving at the source eNB 120 during the outage period will either be dropped or forwarded to the target eNB 130, where the target eNB will send the packets to UE 140 when it is reconnected. At the end of the HO execution phase the DL path is switched from the source eNB 120 to the target eNB 130, and the UE 140 context is released in the source eNB 120.
The already mentioned IRAT HO may occur when a UE moves from the coverage of one RAT into the coverage of an alternative RAT. The IRAT HO mechanisms can also be used for the purpose of balancing the load between two RATS and systems. The IRAT typically comprise phases which correspond to the phases mentioned above with reference to the RAT HO.
The measurement report is configured to UEs by the mobile network, e.g. the ENB of an LTE network. Such reports may be event triggered or triggered on a periodical basis. For an event triggered report, the trigger criteria are configured by the network. By way of example, a source or serving eNB configures UMTS measurements in a UE by providing a so called “RRC Connection Reconfiguration” message to the UE, where the required thresholds for measurement activation are included in the message. The mentioned message also contains the UMTS frequency to be measured by the UE. Once the activation criteria have been fulfilled, the UE measures the UMTS frequency and responds a RRC measurement report, which corresponds to the measurement report, to the eNB. The eNB then takes a decision to perform an IRAT HO to UMTS based on the received report. It is also possible for the network to order an RRC measurement report from a UE by transmitting such a command to the UE. Once an IRAT HO decision has been made, the eNB will continue to perform HO preparation and HO completion.
A release of radio bearers providing a radio connection between a UE and an eNB can have multiple reasons. One release can be due to user inactivity, after expiry of an inactivation timer; a release can be initiated by a mobile user of a UE, or a release can be the result of a successful RAT HO or IRAT HO. However, there can also be abnormal releases, which may also be referred to as drops or session drops, due to e.g. low radio quality, either in the DL or UL direction; transport link problems in the eNB or failed HO.
Unexpected session drops may seriously impact the Quality of Experience (QoE) of mobile users, mainly for bearers carrying a real-time service, such as e.g. Voice-over-IP (VoIP). One of the most important features expected for bearers experiencing a drop is retainability, which is defined as the ability of a UE to retain the bearer once connected, for a desired duration. Retainability Key Performance Indicators (KPI) may be measured in different ways, such as e.g. by measuring the number of dropped sessions normalized with the total number of sessions (drop ratio). In order to improve retainability, there are several ways to optimize the radio network. One way is to ensure sufficient coverage in the server geographical area and also to apply Self-Organizing Network (SON) functions to automatically optimize for coverage and interference. Another way to optimize the network is to fine-tune the handover procedure in order to avoid situations where a HO is initiated too late, i.e. to avoid that a session is dropped before HO execution, that the HO is too early and the session is dropped because the HO fails, or the signal of the target cell is too weak. Such an optimization can be achieved by proper HO parameter settings.
One of the most important purposes with mobile network HO is to avoid drops. A conventional HO can be triggered by low received signal strength from a serving cell which is specified by pre-configured fix thresholds, such as e.g. thresholds referred to as event A2/A3/B2 thresholds applied in LTE networks. Even though there is a high positive correlation between poor radio signal quality and a drop, there is a large uncertainty to predict drop based only on a comparison between the signal quality and fixed thresholds. The exact radio quality level that will eventually cause a drop normally varies in different geo-locations, under different user mobility conditions and with different UEs or terminal types. It is a very difficult, if not impossible, task to optimize the applied thresholds for all occurring variances. Moreover, apart from insufficient received signal strength, drops may be caused by other factors, such as e.g. radio interference, uplink signal to noise ratio, or even measurement problems at the UE. Thus, it might be that one or more of these factors cause a session drop even if parameters to trigger a HO still do not reach a certain threshold value.
According to another example, referring to an IRAT HO from a newly deployed RAT, such as e.g. a HO from LTE to a legacy RAT e.g. GSM, many operators have a policy to keep the UEs in the more advanced RAT unless a drop is going to happen, in order to fully exploit the advantage of the new RAT. In such a case, it may be difficult to set the radio quality threshold to an appropriate level, just high enough to avoid a drop. A conservative threshold is usually configured, with the consequence that many UEs are handed over to the legacy RAT too early.